25 key counties to watch tonight

11/6/12 2:42 PM EST

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Forget about a national election. Forget about the swing states.

The presidential election will be won and lost in a relatively narrow band of key counties within the swing states – some competitive, some partisan strongholds. That’s why Jefferson County, Colo., (pop. 540,000) matters more than the most populous county in the nation, Los Angeles County, Calif. (pop. 9.9 million). And why two Wisconsin counties — Dane (pop. 496,000) and Waukesha (pop. 391,000) — are more relevant to the presidential outcome this year than the behemoth to their south, Illinois' Cook County (pop. 5.2 million).

Here’s a list of 25 of the most important counties tonight across the swing state map.

Colorado

Jefferson County

Republicans have a registration advantage over Democrats in this Denver suburb, but there are almost as many unaffiliated voters as Republicans. It’s a reflection of a swing county that was once a GOP stronghold but in recent years has drifted leftward. While Jefferson went for George W. Bush in 2004, since then a stream of Democratic statewide candidates have carried the county, including Barack Obama, who won by 9 points there.

Arapahoe County

Another suburban Denver locale, Arapahoe often moves in tandem with Jefferson County — both were once solidly Republican, moved toward the center, voted narrowly for George W. Bush in 2004 and then gave Barack Obama even bigger margins in 2008.

El Paso County

Home to Colorado Springs, with its politically engaged evangelical community and heavy military influence, El Paso County is a conservative stronghold where Republicans outnumber Democrats nearly 2-to-1. Romney will need to turn in a big number out of El Paso — now the state’s most populous county — to capture Colorado, and must outperform John McCain, who ran well below the county’s typical landslide GOP presidential margins.

Florida

Duval County

A reliable GOP performer over the years, Jacksonville’s Duval County nearly went for Obama in 2008 — in large part due to high African-American turnout. Republicans need a Romney result in the mid-to-high 50s to help balance out Democratic votes elsewhere in the state.

Hillsborough County

Tampa’s Hillsborough County is one of the nation’s top bellwethers: Since 1960, no presidential candidate has won Florida without carrying it.

That’s why few places have gotten as much care and attention as Hillsborough this year. Not only have the candidates made several appearances, the GOP held its national convention in Tampa this summer. In 2008, both the McCain and Obama campaigns paid close attention to the county, but Obama won by 53 percent to 46 percent — the exact same margin that Bush won by in 2004.

Miami-Dade County

Obama needs to roll up the score in South Florida but particularly in Democratic Miami-Dade, the state’s most populous county. Romney’s performance with Hispanic voters will be important in heavily Cuban-American Miami-Dade, where there are more than a quarter-million Hispanic Republicans.

Orange County

At the other end of the I-4 corridor from Tampa, Romney must improve on McCain’s dismal performance in Orlando’s Orange County, which wasn’t even close in 2008 after being decided by razor-thin margins in 2000 and 2004. But the growth of the non-Cuban Hispanic population in Orange County and neighboring Osceola County — most notably, the Democratic-voting Puerto Rican population — has altered the political equation over the past decade.

South Florida — namely populous Broward, Miami-Dade and Palm Beach counties — turned in big margins for Obama in 2008, which makes the I-4 corridor between Tampa and Orlando all the more important.

Iowa

Dallas County

A suburb of Des Moines, Dallas County is Iowa’s fastest-growing county. If Romney’s not winning in a place McCain won in 2008, he’s not going to win Iowa.

Dubuque County

Northeastern Iowa is the most heavily Roman Catholic part of the state. Obama won Democratic Dubuque County 60-39 in 2008, but Republicans expect Romney and running mate Paul Ryan to cut into Obama’s margins among Catholics as a result of tensions surrounding the federal mandate to cover contraceptive services in health plans.

Johnson County

A Democratic stronghold that’s home to Iowa City and the University of Iowa, the Obama campaign needs a big turnout in Johnson County to win roughly two-thirds of the vote.

Polk County

Polk County (Des Moines) is Iowa’s most populous by a wide margin; Democrats have a 39-31 percent registration advantage. Romney doesn’t have to win the county, but keeping Obama to the low 50s is imperative.

NevadaClark County

Democratic Clark County (Las Vegas), the state’s population hub, is where two-thirds of the Nevada vote was cast in 2008. There’s a significant Mormon population there, which should boost Romney, but the key for Republicans is holding down Obama’s victory margin.

Washoe County

The second-most populous county in the state and home to Reno, once-reliably Republican Washoe voted narrowly for Bush in 2004. Four years later, however, Obama carried it by 12 points. For Romney to win the state, he must win Washoe and Nevada’s rural counties to overcome Obama’s expected Clark County advantage.

New Hampshire

Hillsborough County

If Mitt Romney is going to carry New Hampshire, he’s going to need Hillsborough, where more than a quarter of the state vote was cast in 2008. Its biggest cities are familiar datelines in presidential election years — Manchester and Nashua. This was a Bush county in 2000 and 2004 — narrowly — but Obama pulled out a 4-point win here in 2008.

North Carolina

Mecklenberg County

Democrats held their national convention in Charlotte for a reason — this county, which has more voters than any other in the state, is critical to victory. In 2008, powered by strong African-American turnout, Obama crushed McCain in Mecklenberg by a stunning 62-37 margin — a full 10 percentage points above John Kerry’s 2004 performance.

Wake County

Though Bush carried it twice, Obama blew out the doors in 2008 in Democratic-trending Wake County: He won 57 percent among the affluent and educated Research Triangle electorate. It’s the state's second-most populous county after Charlotte's Mecklenburg County, so Romney must take a bit out of Obama’s 2008 margin to be competitive.

Ohio

Cuyahoga County

If you could know the results of just one Ohio county on election night, Cuyahoga would be your choice. Home to Cleveland, the county is a Democratic stronghold where heavy turnout is essential to Democratic statewide hopes. Four years ago, Obama obliterated McCain there, 69-30, an impressive result even by the county’s standards.

Franklin County

Increasingly Democratic, Columbus’s Franklin County is Ohio’s second-largest population hub after Cuyahoga County. It voted narrowly for Al Gore (49-48), comfortably for Kerry (54-45) and then strongly for Obama (60-39).

Hamilton County

An explosion in African-American turnout in 2008 helped deliver Cincinnati’s Hamilton County to Obama, making him the first Democratic presidential nominee since LBJ to carry the population hub of southwest Ohio.

Romney needs to win here to offset Democratic margins coming out of Cuyahoga County, a goal that’s within reach since both GOP Gov. John Kasich and Sen. Rob Portman, a Cincinnati native and a key Romney surrogate, carried the county in 2010.

Virginia

Henrico County

Like many other suburbs of the Eastern Seaboard, Henrico County is a one-time GOP stronghold that evolved over the years into a battleground.

Over the past 12 years, it voted for George W. Bush twice, for former GOP Sen. George Allen in his unsuccessful 2006 reelection bid and for Republican Bob McDonnell for governor in 2009. But Henrico also voted for Democrats Mark Warner and Tim Kaine for governor and for Barack Obama in 2008.

African-American turnout here and in the Norfolk-Virginia Beach-Newport News area is essential for an Obama victory.

Prince William County

Once reliably and strongly Republican, fast-growing, exurban Prince William County in Northern Virginia has become more competitive over the past decade as it’s grown more diverse. Both 2008 presidential campaigns paid close attention to the county, but on Election Day, Obama won by a surprisingly strong 58-42 score. A Romney victory in Virginia likely begins with a solid showing here; since the Virginia polls close at 7 p.m., it will offer an early indicator of Romney’s suburban and exurban strength.

Wisconsin

Dane County

Reliably liberal and the second most populous county after Milwaukee County, Madison’s Dane County is Wisconsin’s partisan counterbalance to GOP stronghold Waukesha County.

Dane regularly delivers huge Democratic margins, so turnout here is important to the president’s chances of carrying the state for a second time — in 2008, Obama won Dane with 72 percent of the vote.

Waukesha County

If Romney isn’t winning two-thirds of the vote in this GOP bastion, he’s not winning Wisconsin. Like Gov. Scott Walker, who’s twice won more than 70 percent here, Romney needs to roll up big margins here in Wisconsin’s third-largest county, just west of Milwaukee. Without a strong Waukesha performance, it will be almost impossible to offset the Democratic margins coming out of Milwaukee and Dane counties.

Michigan

Oakland County

Michigan is a longshot for Romney, to be sure. To have any chance of winning, he’ll need a strong performance in this historically Republican suburb of Detroit. Romney begins with at least one advantage: He grew up there, and his father served as the state’s governor.

The state’s second-most populous county after Detroit’s Wayne County, Oakland has trended Democratic, but Romney is the kind of GOP nominee who could get traction in the white-collar suburbs here. Some recent polls have Romney winning Oakland and performing well in neighboring Macomb County, both of which are necessary elements of a Romney victory.

Pennsylvania

Bucks County

If Obama’s winning here, chances are he’s also running well across the Philadelphia suburbs. With the big four suburban counties — Bucks, Chester, Delaware and Montgomery — in his pocket and an expected big Democratic margin out of Philadelphia, there’s almost no chance the president will lose the state.

But Republicans are convinced they have an opportunity in the historically GOP-oriented Philly suburbs, which is why Romney appeared in Bucks County on Sunday, just two days before Election Day.